The present invention relates generally to polyester fibers and more particularly, in preferred embodiments, to uncrimped, short-cut polyester fibers suitable for incorporation into wet laid non-woven products.
Polyester fibers and fiber products for use in textile applications are well known in the prior art. Typically, such polyester fibers are made from polyethylene terephthalate (xe2x80x98PETxe2x80x99) polymers by way of multi-step, spin/draw processes. Such processes generally include extruding the PET into a multi-filament tow, drawing the tow of filaments to somewhere between 1.5 and 4 times its original length, and annealing or heat setting the filaments within the tow.
In the production of wet laid non-woven polyester fiber products, the tow is cut into relatively short lengths after being annealed. The short length fibers are dispersed into water and then spread upon a screen. After the water is drained or otherwise removed, the fibers dry to form a wet laid non-woven mat composed of short, intertwined polyester filaments.
The use of polyester fibers for the production of wet laid non-woven products highlights two shortcomings in the traditional production of polyester fiber. First, the individual filaments within the tow tend to adhere to one another and clump together as a result of typical drawing and annealing processes. The clumping is very problematic in the production of wet laid non-woven material since the quality of the non-woven product depends heavily upon the degree of dispersion of filaments within the water.
Second, the individual filaments within the tow tend to deform, or curl, when separated from the other filaments. The deformation occurs due to uneven shrinkage of different filaments within the tow which results from non-uniform annealing of the polyester filaments during the annealing phase of fiber production. Filament deformation adversely affects the production of wet laid non-woven materials because the cut deformed fibers do not intermesh properly when laid upon a screen, resulting in a weakened wet laid non-woven material.
Previous attempts to solve the clumping problems with fiber used in wet laid non-woven. materials have utilized lubricants and other additives to promote the dispersion of the hydrophobic polyester filaments in water. For instance, Shiffler et al., U.S. Pat. No. 5,145,622 discloses a method for improving the dispersibility of polyester fibers by treating them with caustic. In general, the fibers are described as being treated with an appropriate coating as are disclosed, for example, in Hawkins, U.S. Pat. Nos. 4,137,181; 4,179,543; and 4,294,883 and also in U.S. Ser. No. 842,789 filed Mar. 27, 1986 in the names of van Issum and Schluter which discloses the use of a synthetic co-polyester of polyethylene terephthalate units and poly(oxyalkylene) groups derived from a poly(oxyalkylene) glycol having an average molecular weight in the range of 300 to 6,000 as disclosed, e.g. in McIntyre et al., U.S. Pat. Nos. 3,416,952; 3,557,039 and 3,619,269 referred to therein. Other useful segmented co-polyesters are disclosed in Raynolds, U.S. Pat. No. 3,981,807.
Shiffler et al. ""622 uses a commercial water dispersible coating (50/50 mixture of potassium salt of mono and diacid phosphate esters of lauryl alcohol/tallow alcohol ethoxylated with 25 moles of ethylene oxide) on fibers having filaments with round and scalloped-oval cross-sections where a higher level of water-dispersible coating was used to offset the scalloped oval""s approximately 13% higher surface area. The disclosed coating provided the fibers with favorable dispersion characteristics though the utilization of extended cross-sections and mild crimping taught by Shiffler ""622 are not applicable to fibers for use in wet laid non-woven materials.
Similar coatings, which promote dispersion of the short fibers within a water bath, are found in Ring et al., U.S. Pat. No. 4,007,083; Hawkins, U.S. Pat. Nos. 4,137,181; 4,179,543; and 4,294,883; and Viscose Suisse, British Pat. No. 958,350; as well as U.S. Pat. No. 4,713,289 and U.S. Pat. No. 4,707,407. It is noted in the ""289 patent that polyester fibers are naturally hydrophobic, so it is necessary to apply a suitable coating to the polyester to overcome the inherent hydrophobic character of the polyester fiber without creating foam or causing the fibers to flocculate.
It is the lubricants and other surface treatments that have distinguished water-dispersible polyester fiber from more conventional polyester fiber, rather than any inherent characteristic of the polyester itself. The prior art has not addressed the effect of actual fiber production on the eventual dispersion of filaments within a water slurry for production of wet laid non-woven materials.
Similarly, traditional methods of annealing polyester fiber do not address the problems of uneven annealing within the tow which causes clumping of chopped filaments. Traditional methods of annealing polyester fibers tend to promote clumping and adhesion between the filaments of the tow. The clumping of the fibers is undesirable because it limits the dispersibility of the fibers within the liquid medium, resulting in the formation of non-uniform wet laid non-woven mats. The precise cause for adhesion is not well understood, but is believed to result, in part, from the sintering of individual filaments to one another during conventional processing, especially during heat-treatment.
The annealing of polyester fiber, and the associated minimization of fiber shrinkage, has conventionally been accomplished by winding the drawn polyester tow around a series of heated rollers. The heated rollers anneal the fibers at a pre-selected temperature. A problem with using heated rollers for annealing polyester fibers is that the rollers only contact a limited number of the polyester filaments within the tow during each pass over a roller, resulting in uneven annealing of the filaments within the tow. Also, the heated roller only contacts one side of the tow during each pass over a roller, with the tow alternately wound through a series of rollers in an attempt to anneal all sides of the tow evenly. The uneven and non-uniform annealing of the fiber results in a fiber which tends to curl. Such unintended deformation of the fibers is detrimental to the production of wet laid non-woven materials.
Many advances having favorable results have been made in the art of heatsetting crimped polyester fiber, but few advances have been made in favorable heatsetting methods for non-crimped fibers used in the production of wet laid non-woven material. At this point, it should be noted that methods of producing uncrimped fibers for use in wet laid non-woven materials are analogous to, but very distinct from methods for producing fibers which will be crimped.
As mentioned above, the production of a high quality wet laid non-woven material depends on the production of polyester fibers having filaments which do not clump together when dispersed in a liquid medium and which do not deform once separated from the tow. Both clumping and deformation depend on the manner in which the fibers are drawn, annealed, and treated after annealing.
The quality of crimped polyester fibers, on the other hand, does not vary depending on clumping or deformation. Crimped fibers are used mostly for production of woven and knit textiles. Crimped fibers are traditionally extruded, drawn, and annealed using the same methods as fibers for use in wet laid non-wovens, but fibers produced for woven and knit materials are subsequently mechanically crimped, cut, carded, and then spun into thread, either alone or in combination with cotton or other fibers. Filament adhesion has little or no effect on a crimped fiber because the step of mechanically crimping the fiberbreaks apart any adhered filaments. The action of carding the crimped fiber further separates any clumped filaments from one another.
Steam treatment has been used in place of heated rollers for the annealing of polyester fibers which are later crimped. However, steam treatment has not been used to reduce clumping within a non-crimped polyester tow during annealing. For instance, U.S. Pat. Nos. 4,704,329 and 4,639,347 to Hancock et al. and corresponding European Patent No. 0125122 describe a method of utilizing saturated steam to anneal drawn polyester filaments in the production of crimped fiber, with the steam-annealed filaments having an improved balance of strength and shrinkage properties. However, Hancock ""329 utilizes the steam process in the production of crimped fibers, and therefore does not address the problem of clumping in fibers for use in wet laid non-wovens.
Another steam treatment device is described in German Patent Specification DE 195 46 783 C1, in most detail in connection with FIG. 4 thereof. The device disclosed in the ""783 document includes an expansion nozzle feeding a treatment channel wherein the steam accelerates to supersonic speed. The steam decelerates to subsonic speed in the treatment channel before encountering a second nozzle which again accelerates the steam to supersonic velocity. The patent does not discuss clumping within the tow of fiber.
Another method for steam treating polyester fibers is disclosed in U.S. Pat. No. 3,452,132 to Pitzl, wherein a method of heat-treating polyethylene terephthalate yam by applying a steam jet thereto is described. Pitzl impinges the steam jet upon a tow in order to separate the filaments within the bundle and to heat the filaments somewhat instantaneously so that the tow may be uniformly drawn. Pitzl also notes that steam of increased temperature may be used to anneal the polyester fiber. However, the Pitzl process does not address the problems of clumping during the annealing process step, and further involves the use of steam in a combined drawing and annealing process, requiring specialized drawing equipment as well as specialized annealing equipment.
Despite advances in annealing and treatment of polyester fibers, the production of high-quality wet laid non-woven materials is still hampered by the problems associated with polyester fiber deformation and clumping. It has been found in accordance with the present invention that it is possible to make readily dispersible, low shrinkage, uncrimped short cut fibers that overcome the problems of deformation and clumping when used in the production of wet laid non-woven materials.
There is provided in a first aspect of the invention, low-shrinkage, short-cut polyethylene terephthalate (xe2x80x98PETxe2x80x99) fibers exhibiting dispersibility suitable for incorporation into wet laid non-woven products. The invented fibers have a hot-air shrinkage value of less than about 10 percent and a length of less than about 3 inches. Further, the invented fibers exhibit a dispersion index of less than 5. The fibers are prepared by annealing at an elevated temperature of at least about 165xc2x0 C. exclusively through the use of steam. The invented fibers are for use in the production of wet laid non-woven materials, and are therefore uncrimped.
There is provided in another aspect of the present invention, a method of preparing low-shrinkage, short-cut PET fibers exhibiting dispersibility suitable for incorporation into wet laid non-woven products and having a hot air shrinkage value of less than about 10 percent resulting in minimal fiber deformation when dispersed within a liquid medium. The fibers are prepared by annealing at an elevated temperature through the use of steam in a multi-step production process having the following steps: (a) melt-extruding a tow of PET; (b) drawing the filaments to impart orientation thereto; (c) heat-setting the drawn filaments at an elevated temperature through the use of steam, the elevated temperature being at least about 165xc2x0 C.; (d) applying a finish to the tow of filaments; and (e) cutting the tow to a fiber length of 3 inches or less.